The invention relates to a syringe useful for both injection and aspiration.
Various syringe types are available in which the changes in resistance during movement of the plunger (so-called loss of resistance syringe) is used during localization of the epidural area. Suitable syringes comprising a plastic barrel and plunger have in common that little resistance is off, red to the advance movement of the syringe plunger, so that any change in pressure is directly transmitted and can hence be detected.
In addition, the syringe should have the property that during movement of the plunger the friction between the latter and the inner wall of the barrel is constant over the entire plunger stroke, so that an even plunger movement can be achieved by slight pressure on the plunger itself.
A syringe consisting of plastic and having these properties, intended for once-only use, was developed by Portex. This is a three-part syringe, the main advantage of which lies in the high dimensional accuracy of plunger and barrel, thereby permitting a good pressure transmission with low friction. As a result of the plunger form, however, this syringe could only operate effectively during injection, since during sucking the gasket between the plunger and the inner wall of the barrel only had an inadequate effect, such that when the plunger was retracted the vacuum built up was insufficient to suck in fluid.
A further syringe having the properties as described at the outset has been developed by Braun. However, this syringe revealed the same drawbacks during sucking as the Portex syringe, i.e. use in sucking applications was not generally possible.
A further syringe is described in FR 1,048,267. The gasket shown in FIG. 8 comprises three sections, an innermost of which has a rectangular geometry and extends over the full width of a syringe plunger groove holding the gasket. According to FIG. 7, the gasket can comprise a large outer section of rectangular cross-section pressing against the barrel inner wall of the syringe and a small web-like inner section. The gasket here is fairly rigid and does not provide an adequate sealing effect during injection or sucking.
CH 286 277 describes an injection syringe composed of an outer salient section in a wide-area contact with the inner wall of the barrel of the syringe and of an inner membrane-like section surrounding the syringe plunger.
A gasket for an injection syringe in accordance with FR 1,108,413 comprises an outer section of droplet shape that merges into an inner salient section via a web. Both the outer and the inner sections are in wide-area contact with the barrel inner wall or the plunger of the syringe.
A syringe having an O-ring disposed in an all-round groove of the syringe barrel is known from U.S. Pat. No. 4,632,672.
DE 1,566,602 describes an injection syringe consisting of plastic and having the drawbacks described at the outset.
A flat gasket is used in accordance with DE 2,024,117 for sealing a plunger in relation to a barrel of an injection syringe consisting of plastic.
Further syringes are described in U.S. Pat. No. 1,154,677, U.S. Pat. No. 2,578,814, EP 0,102,070 A2, FR 4 06,988, GB 1,179,487 and DE 2,451,398 A1.
The problem underlying the present invention is to develop a syringe of the type stated at the outset such that it can be used for both injection and sucking, with low friction losses occurring at the same time while the plunger is being moved.